Phased array antennas have the important ability of beam steering without mechanical actuators. This feature is highly desirable for applications such as spacecraft, air craft, and mobile platforms where size and mass are restricted. The direction of a microwave (or millimeter wave) beam radiated from a phased array antenna is generally controlled by the relative phase distribution of microwave signals emitted by regularly spaced radiating elements. For a phased array of a wide instantaneous bandwidth, adjusting only the relative phase is not sufficient and so a relative time delay adjustment of the radiating elements must be introduced to avoid the beam pointing error known as squint, which results from the modification of the antenna phase pattern with changing frequency.
Conventional electronic beam forming systems for generating and delivering the requisite time delay and phase information are generally bulky, lossy, inefficient, and of narrow bandwidth. On the other hand, photonic beam forming offers the advantage of high packing density, wide signal bandwidth, light weight, immunity to electromagnetic interference, and remoting capability via optical fiber. Consequently, it has been under intensive investigation in the past few years and many photonic beam forming systems have been proposed and demonstrated. Photonic beam forming network use a lightwave carrier for the electrical signals of the radiating elements of the phased array, and provides the necessary time delay and phase information for beam steering.
For airborne and space-based phased arrays operating at mm-wave frequencies (20 GHz and above), the arrays are usually two-dimensional and a large number of array elements, typically a few thousand, are used. This requires that the beam forming network be two dimensional and have a very high packing density. In addition, the beam forming network must be reversible so that it can be used for both antenna transmitting and receiving. Furthermore, the total delay achievable of the delay network must be sufficiently large so that the maximum scanning angle of the phased array is adequate. Finally, as will be shown later, the delay resolution (the minimum step of delay change) must be fine enough (much less than the wavelength of the signal) to ensure that the angular resolution of the beam scanning is sufficient.
None of the proposed photonic beam forming networks to date meet all of the above requirements. The operation frequencies of the beam forming networks based on acousto-optic modulators (E. Toughlian and H. Zmuda, "A photonic variable RF delay line for phased array antennas," J. Lightwave Technol., vol. 8, pp. 1824-828, 1990) are limited to below 5 GHz and suffer from poor delay resolution, and therefore not adequate for mm-wave phased arrays. Path-switching time delay devices based on guided wave optics (C. T. Sullivan, S. D. mukherjee, M. K. Hibbs-Brenner, and A. Gopinath, "Switched time-delay elements based on AlGaAs/GaAs optical wave-guide technology at 1.32 mm for optically controlled phased array antennas," SPIE Proceedings, vol. 1703, pp. 264-21, 1992) are complicated, and are characterized by high loss, high cost, poor delay resolution, and one-dimensional geometry. The free-space path-switching time delay device (N. A. Riza, "Transmit/receive time-delay beam forming optical architecture for phased array antennas," Appl. Opt., vol. 30, pp. 4594-4595, 1991) shown in FIG. 1A is a two dimensional device of high packing density, and operates at high frequency with sufficient total delay. However, as shown in FIG. 1B, the delay resolution of the device is limited by the size of the vertical dimension d of the two dimensional delay array and equals to 2dn, where n is the refractive index of the required polarization beam splitting cube. For a d of 10 cm and a n of 1.5, the resulting delay resolution is 30 cm and is much too large for mm-wave antennas. In addition, presently the path-switched true time delay has a non-optimized design, making it bulky, expensive, and difficult to manufacture.
Even for narrow bandwidth phased arrays where true time delay is not necessary, a compact, two dimensional, and programmable phase shifter with high phase-shift resolution is highly desirable. Such a phase shifter can reduce the size and weight, and increase the pointing accuracy of the phased array radar.
Another important application of two dimensional true time delay device is in transversal filters (B. Moslehi, K. Chau, and J. Goodman, "fiber-optic signal processors with optical gain and reconfigurable weights," Proc. 4th Biennial Department Of Defense Fiber Optics and Photonics Conf., McLean, Va. 1994, pp. 303-309 and D. Nortton, S. Johns, and R. Soref, "Tunable wideband microwave transversal filter using high dispersive fiber delay lines," Proc. 4th Biennial Department Of Defense Fiber Optics And Photonics Conf., McLean, Va., 1994, pp. 297-301). In such a filter, a microwave or mm-meter wave signal is splitted into many branches and then recombined after the signal in these branches experiences different delays. For a certain set of delays, only the signal with a right frequency will add in phase and exit the beam combining junction with minimum loss. Other frequencies will destructively interfere and suffer severe loss--a bandpass filter is formed. By changing the delay arrangements, the center frequency of the pass band will also change, creating a dynamically tunable filter often referred to as transversal filter. Studies indicate that the bandwidth of the filter is inversely proportional to the number of branches and the frequency tuning resolution is proportional to the delay resolution of the branches. Therefore, a compact, two dimensional, and programmable true time delay with high delay resolution is ideal for constructing such a filter.
Yet another application of a variable delay line with high delay resolution is in optical interferometry, and in auto- and cross-correlation measurements of optical pulses. Presently, variable delay is accomplished by the combination of various forms of mechanical translation and is fine tuned by piezoelectric transducer. Because such a delay line involves mechanical moving parts, it is generally bulky, heavy, difficult to align, and less reliable. In addition, because the piezo-electric transducer suffers from hysteresis and temperature dependent drift, active control using feedback servo loop is required, resulting in a complicated system.